Channel coding is generally employed in digital magnetic recording to match certain properties of the coded sequence to the channel characteristics of the recorder. Since magnetic recorders are incapable of reproducing very low frequencies or direct current (dc) content, dc-free channel codings, such as 8/10 block code, modified Miller code (M.sup.2), interleaved NRZI, and tri-level code, have primarily been employed in digital video tape recorders (VTRs).
A new dc-free channel code for digital magnetic recording was reported by Hirofumi Yoshida, Toshiyuki Shimada, and Yoshitaka Hashimoto in an article entitled "8-9 Block Code, A DC-Free Channel Code for Digital Magnetic Recording," in the Sept. 1983 issue of the SMPTE Journal, pp. 918-922. In this article, the authors point out that channel coding methods are considered to be the combination of two processes. First is the transformation of m-bit data words to n-bit code words which are concatenated to form the recording bit sequence. Second is the modulation of the code words by the fundamental modulation methods, NRZ or NRZI (nonreturn to zero or nonreturn to zero inverse). If the m-bit data words are transformed to the n-bit code words using a prearranged lookup table or dictionary, the coding method is called block coding.
Several terms must be defined for an understanding of the coding scheme. The ratio of m/n is called the code rate. The bit interval of source data (T) is usually taken as a reference, so that each coded data rate is then normalized to the source data rate. The minimum and maximum time intervals between magnetic transitions are denoted by T.sub.min and T.sub.max, respectively. The digital sum variation (DSV) is a running integral of the recording waveform. In computing DSV, the binary levels are assumed to be .+-.1. If the DSV of the code can grow indefinitely, the code has dc content which can't be recorded. If the DSV is bounded, the code is dc free. The code word digital sum (CDS) is the digital sum variation from the beginning to the end of a code word.
Since the number of bits in the code words of 8/9 NRZ is odd, it is impossible to have zero CDS code words. It is possible, however, to make the 8/9 NRZ dc free by constructing a one to two correspondence. First 512 (2.sup.9) code words are divided into two groups of 256 code words each of which have opposite CDS polarities. Then, while observing the DSV of the coded sequence, the code words from the appropriate groups are selected to make the DSV bounded.
The problem with this coding scheme is that given arbitrary data, there is no unique sequence of bits to synchronize the reproduction process. One current synchronizing method uses a probabilistic process wherein the data is scrambled before the 8/9 coding and a fixed, unscrambled sequence of 9 bit bytes is chosen as the synchronizing pattern because it is unlikely to be reproduced as data. This pattern is then detected upon reproduction to provide the sync indication. Because there is at least the possibility of an input word being reproduced as a sync word, this method of generating a sync pattern is undesirable.
What is needed is a unique sync word pattern which can be detected regardless of the input data and which retains the dc-free nature of the code.